1. Field of the Invention
The present invention relates to a reflection-type or transflective-type liquid crystal display device which performs display by utilizing reflected light.
2. Description of the Related Art
Liquid crystal display devices (LCDs) are often transmission-type LCDs which utilize backlight from behind the display panel as a light source, reflection-type LCDs which utilize reflected external light as a light source, and transflective-type LCDs which utilize both reflected external light and backlight light sources. The reflection-type LCD and the transflective-type LCD are characterized in that they have smaller power consumptions than that of the transmission-type LCD, and their displayed images are easy to see in a bright place. The transflective-type LCD is characterized in that the displayed images are easier to see than that of the reflection-type LCD, even in a dark place.
FIG. 13 is a cross-sectional view showing an active matrix substrate 100 in a conventional reflection-type LCD (e.g., Japanese Laid-Open Patent Publication No. 9-54318).
As shown in FIG. 13, an active matrix substrate 100 includes an insulative substrate 101, as well as a gate layer 102, a gate insulating layer 104, a semiconductor layer 106, a metal layer 108, and a reflective layer 110, which are stacked on the insulative substrate 101. After being stacked on the insulative substrate 101, the gate layer 102, the gate insulating layer 104, the semiconductor layer 106, and the metal layer 108 are subjected to etching by using one mask, and are thus arranged to have an island-shaped multilayer structure. Thereafter, the reflective layer 110 is provided on this multilayer structure, thereby providing a roughened reflection surface 112. Although not shown, transparent electrodes, a liquid crystal panel, a color filter substrate (CF substrate), and the like are formed above the active matrix substrate 100.
In the aforementioned active matrix substrate 100, portions of the reflective layer 110 are arranged so as to reach the insulative substrate 101 in portions where the gate layer 102 and the like are not formed (i.e., portions between the islands, hereinafter referred to as “gap portions”). Therefore, in the gap portions, the surface of the reflection surface 112 is recessed in the direction of the insulative substrate 101, thus forming a surface having deep dents (or recesses).
In the reflection-type liquid crystal display device or the transflective-type liquid crystal display device, it is necessary to allow incident light entering from various directions to be reflected by the reflection surface 112 more uniformly and efficiently over the entire display surface in order to perform bright display by utilizing reflected light. For this purpose, it is better if the reflection surface 112 is not completely planar but has moderate roughness.
However, the reflection surface 112 of the aforementioned active matrix substrate 100 has deep dents. Therefore, light is unlikely to reach the reflection surface located on the bottoms of the dents, and even if at all light reaches there, the reflected light thereof is unlikely to be reflected toward the liquid crystal panel, thus resulting in a problem in that the reflected light is not effectively used for displaying. Furthermore, there is a problem in that, since many portions of the reflection surface 112 have a large angle relative to the display surface of the liquid crystal display device, the reflected light from those portions is not effectively utilized for displaying.
FIGS. 14A and 14B are diagrams showing a relationship between the tilt of the reflection surface 112 and reflected light. FIG. 14A shows a relationship between an incident angle α and an outgoing angle β when light enters a medium b having a refractive index Nb from a medium a having a refractive index Na. In this case, according to Snell's Law, the following relationship holds true:Na*sin α=Nb*sin β
FIG. 14B is a diagram showing a relationship between incident light and reflected light when incident light entering perpendicularly to the display surface of an LCD is reflected from a reflection surface which is tilted by θ with respect to the display surface (or the substrate). As shown in the figure, the incident light perpendicularly entering the display surface is reflected from the reflection surface which is tilted by angle θ with respect to the display surface, and goes out in a direction of an outgoing angle φ.
According to Snell's Law, the results of calculating the outgoing angle φ according to Snell's Law with respect to each angle θ of the reflection surface are shown in Table 1.
TABLE 1θφ90 − φ009026.00612183.99388412.0496777.95033618.1718171.82819824.4221265.577881030.8658859.134121237.5970952.402911444.7655445.234461652.6438237.356181861.8454328.154572074.6185715.3814320.579.7654210.2345820.681.127578.87243220.782.733157.26684820.884.803115.1988820.988.850361.14963720.90589.799140.200856
The values in this Table are calculated by assuming that air has a refractive index of 1.0 and the glass substrate and the liquid crystal layer have a refractive index of 1.5. As shown in Table 1, when the angle θ of the reflection surface exceeds 20 degrees, the outgoing angle φ becomes very large (i.e., 90−φ becomes very small), so that most of the outgoing light does not reach the user. Therefore, even if roughness is provided on the reflection surface of the reflective layer, it is necessary to ensure that the angle θ is 20 degrees or less in greater portions of the reflection surface in order to effectively use the reflected light.
Since the reflection surface 112 of the aforementioned active matrix substrate 100 has many portions which are greater than 20 degrees, reflected light is not very effectively used for displaying. In order to solve this problem, it might be possible to form an insulating layer under the reflective layer 110, and form the reflective layer 110 over the insulating layer. However, in this case, a step of forming an insulating layer, and a step of forming contact holes for connecting the reflective layer 110 to the drains of TFTs in the insulating layer are needed, thus resulting in a problem of an increase in the material and the number of steps.